The utilization of zinc electrodes in rechargeable nickel-zinc electrochemical cells and methods for their preparation are well known. Nevertheless, numerous problems exist involving the design and use of current-producing rechargeable electrochemical cells which derive in part from the phenomena of the redeposition of active materials upon one of the electrodes. Rechargeable electrochemical cells utilizing Zn/ZnO electrodes have heretofore been prone to exhibiting the growth of Zn dendrites during the charging phase of such cells.
For a typical nickel-zinc rechargeable cell, the number of charge/discharge cycles to which a given cell may be subjected will be limited by the above-mentioned dendrite formation problem which severely limits the useful life of such cells by effectively shorting the cell.
The dendrite formation problem in a typical rechargeable nickel-zinc electrochemical cell is understood to be caused by the fact that the zinc goes through a soluble species stage during each charge/discharge cycle. During the discharge period of each such cycle zinc hydroxide is formed.
Zinc hydroxide is highly soluble in alkaline systems, such as the electrolytic environment in which these cells operate, and, therefore, readily goes into solution in such an electrolytic medium. Eventually, the limit of solubility is reached and the zinc hydroxide precipitates out of solution. Upon recharging the cell, zinc is plated out of the saturated solution, thus allowing more zinc hydroxide to enter into the solution.
The problem revolves around the fact that the zinc which comes out of the saturated solution can, and often does, plate out at one point on the electrode surface and, when this occurs, a dendrite eventually forms, shorting the zinc/zinc oxide electrode to the nickel electrode.
A further problem which is encountered involves the fact that a "shape change" occurs in the crystal structure of the zinc which is deposited by plating from the saturated solution onto the surface of the zinc/zinc oxide electrode.
Ordinarily, a fine zinc deposit on the surface of the electrode is necessary in order to maintain a high surface area exposed to the electrolytic solution. When zinc hydroxide comes out of the solution and is subsequently redeposited on the surface of the electrode, as described above, coarse deposits of zinc may be formed which do not possess the requisite high surface area characteristics which are required, thus inherently limiting the useful capacity of the electrode and, therefore, of the rechargeable electrochemical cell.
It has been suggested that relatively high levels of PbO and CdO, in the order of 2 percent by weight PbO and 0.5 percent by weight CdO, have a beneficial effect in reducing the shape change which occurs upon the deposition by plating of zinc on a zinc electrode. See O. Wagner and A. Himy "Substitutes for Mercury in Alkaline Zinc Batteries" (27th Power Source Symposium).
However, neither this nor any other reference of which the applicant is aware makes any mention as to an improvement in the dendrite formation problem referred to above nor in achieving an increase in cycle life for rechargeable electrochemical cells.
It has now been discovered that superior zinc/zinc oxide electrodes for utilization in the construction of rechargeable nickel-zinc electrochemical cells may be prepared by employing a specific type of commercially available ZnO in the manufacture of such zinc electrodes.
It has been found that by employing a ZnO component having a different combination of particle structure and level of impurities present from that of the accepted industry standard, the problem of failure by dendrite shorting is greatly decreased, thus allowing for a ten-fold increase in the number of effective charge/discharge cycles for cells manufactured utilizing such improved electrodes.